The invention relates generally to optical surfaces used in the infrared and visible wavelength regions and more particularly to high quality optical surface and a method of producing high quality optical surfaces.
High quality optical surfaces are used in a number of industries including telecommunications, in the visible and infrared wavelength regions of the optical spectrum. The quality of an optical surface is measured by the flatness, the surface scatter and the reflectivity of the surface.
The achievable flatness of a surface is limited by the Computer Numerical Controlled (CNC) manufacturing process. The typical methods of manufacturing optical shapes in metals include conventional grinding, polishing and CNC manufacture using diamond tools. However, these conventional methods do not lend themselves to the manufacture of complex shapes. The surfaces are uneven and comprise a number of undulations.
The scatter of an optical surface is influenced by the diamond turning process. Conventional diamond turning methods do not produce uniform thickness along the surface. Typical materials used in standard optical shapes include aluminum, copper and nickel. Aluminum generally contains a number of impurities and these impurities are heavily concentrated at grain boundaries. Therefore, diamond turning an aluminum surface usually results in values of about 10 nanometers Root Mean Squared (RMS) for surface scatter. Copper generally yields better resultsxe2x80x94about 5 RMS, however copper has the drawback of being susceptible to corrosion.
A typical method of producing high quality surfaces with an aluminum substrate is to plate the surface with nickel and to diamond turn the nickel. Using this method, typical values achieved for surface scatter are about 5 nanometers RMS. However, a major problem for nickel plated substrates that are diamond turned exists. Since the diamond turning procedure results in a non uniform thickness of nickel and since nickel and aluminum have very different thermal expansion coefficients, the optical surface tends to deform as the temperature changes in the operating environment. This reduces the quality of the surface.
The achievable reflectivity at the operating wavelength is determined by the material properties of the optical surface. Typically gold is applied to the surface of a substrate to increase the reflectivity in the infrared. Gold is usually applied by vacuum depositing methods, which produces generally an uneven surface application. Also, vacuum deposited gold is generally soft gold. This gold layer has to be covered with a protective coating, which reduces the reflectivity of the surface.
Alternatively, it is also known to electroplate gold on to the surface. However, since the surface is generally uneven and comprises a number of undulations, the applied gold layer must be relatively thick to compensate for those undulations and provide a generally even surface. This results in high costs.
Often it is required that the optical surfaces be double sided; that is the optical surface comprise reflective surfaces on both sides of the substrate. Known methods do not lend themselves to producing good quality surfaces that are double sided because of the general unevenness of the substrate layer. When the top layers are diamond turned, they are diamond turned until each respective layer is even. However, the amount of diamond turning necessary to achieve this is generally different for each side. This is important because the differences in the final thicknesses of the layers on each side result in deformation due to temperature changes as described above. Due to the nature of the uneven substrate surface, this problem is inherent.
With the sophisticated uses of computer aided design, the optical surfaces required are becoming more and more complex. Also the tolerance for errors in these surfaces is decreasing. There exists a demand for high precision and high quality optical surfaces in a variety of geometrical shapes.
Therefore, there also exists a need for a method to produce these optical surfaces at an effective cost.
The invention is directed to a method of producing high quality optical surfaces. The invention allows for the production of optical surfaces with complex geometry while retaining the high quality.
According to one aspect of the invention, there is provided a method of producing an optical surface comprising the steps of: providing a substrate, the substrate being stress free; applying an adhesion layer to the substrate, the adhesion layer including a first adhesion layer and a second adhesion layer; applying a top layer to the adhesion layer; and diamond turning the top layer to provide a reflective layer thereon.
According to another aspect of the invention, there is provided an optical surface comprising: a substrate the substrate being stress free; an adhesion layer on the substrate, the adhesion layer including a first adhesion layer and a second adhesion layer; top layer on the substrate; the top layer being diamond turned to provide a reflective layer thereon.
According to another aspect of the invention, there is provided an optical surface produced by a method comprising the steps of: providing a stress free substrate; applying an adhesion layer to the substrate; applying a top layer to the adhesion layer; and diamond turning the top layer to achieve a reflective layer.
According to another aspect of the invention, there is provided a double sided optical surface comprising: a stress free substrate, the substrate comprising a first and second surface; an adhesion layer on each of the first and second surfaces of the substrate, each adhesion layer including a first adhesion layer and a second adhesion layer; and a top layer on each of the adhesion layers, the top layers being diamond turned to achieve reflective layers on each of the first and second surfaces of the substrate.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.